In order to increase the capacity of nonaqueous electrochemical devices, such as lithium ion secondary batteries and electric double layer capacitors, it has been studied to increase the density of the electrodes and increase the amount of active substance per volume thereof. As a binder for electrode substances, a vinylidene fluoride resin has been principally used. In the preparation of an electrode by packing an electrode sheet formed by applying and drying an electrode-forming composition on a metal foil (electroconductive substrate), however, the resultant composite electrode layer is liable to be cracked to separate from the electroconductive substrate or the electroconductive substrate per se is liable to be broken, e.g., when the electrode sheet is wound about an axis at a short curvature radius in the case of a cylindrical battery or the electrode sheet is bent in the case of a rectangular battery.
In some cases, a rubbery polymer such as styrene-butadiene rubber is used as a binder for the purpose of providing a composite electrode layer with flexibility, it is difficult to increase the density because of rubber elasticity when the composite electrode layer is pressed. As for fluorine-containing resins, a soft binder can be provided if vinylidene fluoride-chlorotrifluoroethylene (CTFE) copolymer or vinylidene fluoride-hexafluoropropylene (HFP) copolymer is used to lower the crystallinity of vinylidene fluoride, but it is known that the composite electrode layer is swollen or dissolved with an electrolytic solution in case of an increased CTFE or HFP content.
Use of a rubber-type binder has been proposed (Patent document 1 listed below), but when it is used alone in preparation of electrodes, the powdery active substance is liable to be set in separation in the electrode-forming slurry so that the formation of a stable composite electrode layer becomes difficult. It has been also proposed to blend a vinylidene fluoride resin with a rubbery polymer (Patent document 2 below), but when they are dissolved in an organic solvent such as N-methyl-2-pyrrolidone (NMP) to form a binder, they are liable to cause phase separation in the binder solution, and when the binder solution is further blended with an electrode material such as an active substance to form an electrode-forming slurry, the active substance and the solution components are liable to separate from each other so that the formation of a uniform electrode by application becomes difficult.
While the use of a core/shell-type binder has been proposed (Patent document 3 below), the core formed of PTFE has a high elasticity modulus, so that the formation of a flexible electrode is impossible.
Patent document 1: WO 98/39808A
Patent document 2: JP 09-21337A
Patent document 3: WO 96/12784A